I never realized this mossy big wall in the Dominion Killing Zone in Kassex Hills. It even has a black gates and I did Kassex with few characters so far.
Is this new or has it always been there? Any idea what it might represent?

that thing extends pretty fair. Go to the poi below White Crane Terrace in Lions arch, you'll see they are there too.

That bit actually seems separate from the actual walled-off city. Because if it was part of the wall that encircles the Dominion of Winds, how the hell did karka get to Garrenhoff, and for that matter why is Garrenhoff a port town?

It's more likely that's just a newly erected gate into lands near the Dominion of Winds that the tengu began claiming.

What you saw is probably northern section wall of Dominion of Winds. You can see the western section wall in the easternmost part of Caledon Forest. There is even a bridge over a lake that you can cross over and talk to a few Tengus stationed there.

Really? That's it? I was hoping for something more lore-tastic than that. Oh well, sometimes things stare you right in the face. After you say that, I can see, because you are helping at Tengu there kill off destroyers.

I'd have a somewhat deeper and possibly more cynical way of looking at it - from memory of last time I was there, the region is fairly devoid of cover (or at least was, before the destroyers started breaking up the land) and within clear view of the top of the wall. So at the very least, the 'killing zone' might have been an area which the tengu kept clear of cover that might otherwise assist a potential attacker to reach the wall undetected - thus, serving as the 'killing zone' for anyone seeking to attack the wall. At a greater extreme, there might have been a time when the tengu were so thoroughly isolationist that they'd simply shoot everyone that approached...

The border is pretty weird i think, it looks like the tengu control Sanctum Cay and the D'alessio Seaboard, but they are pretty cut off from each other, right in between them is Claw Island and the Wizard's tower.

That bit actually seems separate from the actual walled-off city. Because if it was part of the wall that encircles the Dominion of Winds, how the hell did karka get to Garrenhoff, and for that matter why is Garrenhoff a port town?

It's more likely that's just a newly erected gate into lands near the Dominion of Winds that the tengu began claiming.

The only problem with that is that it doesn't look any newer than the rest of the wall. But that may not mean anything I suppose. Real people make games, and they have time budgets...